Electrographic apparatus for magnetic printing

ABSTRACT

Electrographic apparatus wherein a rotatable surface is interposed between the recording surface of the apparatus and means is provided for applying printing powder to the rotatable surface, the rotatable surface providing the means to develop a latent image on the recording surface. The thickness of the layer of printing powder is regulated by a plurality of magnetic drums spaced apart from one another by an amount which decreases near to the recording surface.

United States Patent Field of Search 346/74 MP Hodges a Sept. 10, 1974ELECTROGRAPHIC APPARATUS FOR [56] References Cited MAGNETIC PRINTINGUNITED STATES PATENTS [75] Inventor: Robert James Hodges, Cheshunt,3,250,636 5/1966 Wilferth 346/74 MP England 3,370,546 2/1968 Muller346/74 ES 3,419,884 12/1968 Betts et a1. 346/74 ES Asslgneer StandardTelephones and Cables 3,437,074 4/1969 l-lagopian et a1 346/74 MPLimited, London, England [22] Filed. Oct 28 1970 PrimaryExaminer-Bernard Konick Assistant Examiner-Jay P. Lucas [211 App]. No.284,947 Attorney, Agent, or FirmA. Donald Stolzy Related US. ApplicationData [63] Continuation of Ser. No. 770,691, Oct. 25, 1968, [57] ABSTRACTabandoned Electrographic apparatus wherein a rotatable surface isinterposed between the recording surface of the ap- 30 ForeignApplication priority Data paratus and means is provided for applyingprinting Nov 23 1967 Great Britain 53328/67 powder to the rotatablesurface, the rotatable surface 8 1968 Great Britain 415/68 providing themeans to develop a latent image on the recording surface. The thicknessof the layer of print- [52] Us Cl 118/637 346/74 MP ing powder isregulated by a plurality of magnetic [51] d 15/12 Bosh 3/02 drums spacedapart from one another by an amount [58] which decreases near to therecording surface.

1 Claim, 8 Drawing Figures PATENIEB SEP 1 0 I974 SHEET 1 Bf 4 InventorROBERT J. #0055 A Home y PATENIEBSEPIOW 3334.350

SHEET 2 OF 4 I v J5 I I Inventor ROBERT J. #00565 www- A Home yPAIENTEDSEP10I974 3.884.350

SHEU SN 4 Inventor ROBERT J. #00 55 @Mw-l A Home y PAIENIED EPI3.884.350

sass: u or 4 Inventor ROBERT J. #00 55 Aflorne y ELECTROGRAPHICAPPARATUS FOR MAGNETIC PRINTING This application is a continuation of acopending parent application, now abandoned, Ser. No. 770,691, filedOct. 25, .1968, and of the-same title.

The invention relates to electrographic printing and- /or displayapparatus.

Electrographic'printing and/or display apparatus is defined as apparatusof the kind wherein the recording surface of either a print drum or bandis selectively magnetised or electrostatically charged to form a patternor latent image thereon representative of information contained in asignal applied to the apparatus, and wherein the print drum .or band ispassed through or relative to a powder applicator containing a powderthat is attracted to the electromagnetically .or electrostaticallyformed latent image to develop same and to form a powder image which maybe viewed at a display position and/or transferred to a permanent recordby printing means which form part of the apparatus.

In order to achieve a good print out or display, it is necessary tocarefullypowder the latent image so as to achieve a clean background tothe powdered image and to provide a powder image of even powder density.Fluid powder beds have been used to apply the powder directly to thelatent image but difficulties with regard to irregular powder built upon the latent image have been experienced which are in the main due to ajet ting effect occurring in the powder contained in the fluid powderbeds.

The present invention provides an electrographic printing and/or displayapparatus of the kind as hereinbefore defined including a permanentlymagnetised or electrostatically charged rotatable surface interposedbetween'the recording surface of said apparatus and means for applyingprinting powder to said rotatable surface, said rotatable surfaceproviding the means to develop a latentimage or images on said recordingsurface.

The foregoing and other features according to the invention will bebetter understood from the following description with reference to thedrawings accompanying the Specifications,- in which:

' FIG. 1 diagrammatically illustrates a perspective view in part sectionof part of an electrographic printing apparatus and a transfer powderingdevice.

FIG. 2 diagrammatically illustrates a sectioned front view of part of anelectrographic printing apparatus and another transfer powderingarrangement.

FIG. 3 diagrammatically illustrates a perspective view in part sectionof part of an electrographic display apparatus and thetransfer'powdering device shown in the drawing according to FIG. .1,

FIG. 4 diagrammatically illustrates a sectioned front view of anelectrographic printing and/or display apparatus and another transferpowdering device, a

FIG. 5 diagrammatically illustrates a sectioned front view of part ofeither the electrographic printing apparatus shown in thedrawingaccording to FIG. 1 or2 or the electrographic display apparatus shown inthe drawing according to FIG. 3 and an impeller powdering device,

FIG. 6 diagrammatically illustrates a cross-sectioned end view of partof an impeller powdering device.

FIG. 7 diagrammatically illustrates a sectioned front view of part of anelectrographic printing and/or dis- 2 play apparatus and anotherimpeller powdering device, and

FIG. 8 diagrammatically illustrates a plan view of an electrographicdisplay apparatus which utilizes a transfer and/or impeller powderingdevice.

Referring to FIG. 1, part of an electrographic printing apparatus and atransfer powdering device are diagrammatically illustrated therein in apart sectioned perspective view. The transfer powdering device basicallyconsists of a band or roller 1 interposed between a powder reservoir 3and the recording surface 4 of the print drum 5 which forms part of theelectrographic apparatus and which is rotated in the direction of arrowD by a drive shaft 8 attached thereto by means not shownin FIG. 1.

It will be assumed by way of example that the recording surface of theprint drum 5 is selectively magnetized to form the pattern thereontherefore the outer surface 2- of the band or roller 1 will need to bepermanently magnetised in a manner such as to collect thereon an evenlydistributed layer of magnetic printing powder 6 contained in thereservoir 3 when it is rotated therein in the direction of arrow E by adrive shaft 7 which is attached to the band or roller 1 by means notshown in FIG. 1.

The permanently magnetised transfer surface 2 is separated from therecording surface 4 by an amount such that consistent powder transfer tothe latent magnetic image which is formed on the recording surface 4 ofthe print drum 5 by means of the magnetic recording head unit 9, isobtained, i.e., the recording surface 4 of the print drum 5 is arrangedto just contact the outer surface of the evenly distributed layer ofprinting powder 6 on the surface 2 of the band or roller 1 and such thatthe magnetised surface 2 does not have a demagnetising effect on thelatent image. The strength of the magnetic field of the band or roller 1therefore determines the separation distance between the transfersurface 2 thereof and therecording surface 4 since the stronger the Ifield of the transfer surface 2 then the greater the build up of theprinting powder 6 and therefore the gap between the transfer surface 2and the recording surface 4 will need to be larger in order to ensurethe recording surface 4 just contacts the. outer surface of the layer ofprinting powder 6. Therefore, the stronger magnetic field will have theleast demagnetisa- 'tion effect on the latent magnetic image.

Normally, the surface 2 of the roller 1 and the surface 4 of the printdrum 5 would have the same linear velocity but if any irregularitiesexist at the outer surface of printing powder 6 then it may be necessaryfor the surface 2 to have a higher linear velocity than the surface 4.

When the latent magnetic image is powdered with the printing powder 6,the print drum surface 4 comes in contact with a strip of paper 10 whichis moving in the direction of the arrow A at the same linear velocity asthe surface 4 of the print drum 5. The paper strip 10 is dispensed froma roller of paper 13 wound on a shaft 14 which is roatated in thedirection of the arrow B. A pressure roller 11 which is rotated in thedirection of the' arrow C by a driving shaft 12 attached thereto bymeans not shown in FIG. 1, presses the paper strip 10 against the drumsurface 4, and the powdered image is transferred under pressure from thedrum surface 4 to the surface of the paper strip 10. It is the usualpractice in such processes to include a therthe surface of the paperstrip 10.

mal fixing agent, for example, resin or wax, in the powder formulationso that the powdered image may be fixed by the application of heatsubsequent to the formation of the powdered image, therefore the paperstrip after passing between the drum 5 and the roller 11 is passedthrough heating means (not shown in FIG. 1) wherein the powdered imageis thermally bonded to The thickness of the layer of printing powder onthe permanently magnetised transfer surface 2 of the band orroller 1 maybe more accurately controlled in order toeffect better control of thetransfer-of the printing powder6 by utilizing a plurality of bands orrollers 1 interposed between the recording surface 4 of the print drum 5and the powder reservoir 3 which may be arranged for example as shown inthe drawing according to FIG. 2.

Referring to FIG. 2 part of an electrographic printing apparatus andanother transfer powdering arangement are diagrammatically illustratedtherein in a sectioned front view. The transfer powdering arrangement ofthis apparatus utilizes three transfer bands or rollers, i.e., therollers 1, 1A, and 1B interposed between the recording surface 4 and thepowder reservoir 3, the gap between the surface 2 of the roller 1A andthe surface 20f the roller 13 being less than the gap between thestorage is required due to leakage problems associated with the magneticimages.

' 3 FIG. 3 diagrammatically illustrates a perspective view in partsection of part of an electrographic display apparatus and the transferpowdering device outlined in preceding paragraphs-with reference to thedrawing according to FIG. 1. It will of course be appreciated that thetransfer powdering device may also in this application be modified andadapted as outlined in preceding paragraphs. The operation of thisapparatus in basically the same as the apparatus outlined with referenceto P16. 1 except the print out facility is eliminated and the latentmagnetic image which is formed on'the surface of a band 15 by means of amagnetic recording head unit 19 and rendered visible by powdering withthe band or roller 1 in association with the powder reservoir 3 in amanner as previously described, is presented in visible form to anobserver by arranging for that part of the surface 20 of the band 15which ap- 20 pears between the transfer powdering device and an surface2 of the roller 1A and the surface 20f the roller l but the same as thegap between the surface 2 of the roller 18 and the surface 4 of theprint drum 5. The strength of the magnetic field at the surface 2 of theroller 1A is arranged to be greater than the magnetic field at thesurface 2 of the roller 1B but less than the magnetic field at thesurface 2 of the roller 1 therefore the thickness of thelayer ofprinting powder 6 on the surface 2 of theroller 1B i.e., the rollernearest to the recording surface 4 will be accurately controlled, theprinting powder 6 being evenly distributed by the succession'of rollerssince each subsequent roller in the chain of rollers between the powderreservoir 3 and the recording surface 4 reduces the thickness of thelayer of printing powder on the previous roller by a fixed amount andtends to even out-any irregularities at the outer surface of the powderlayers.

Alternatively, the thickness of the layer of printing powder 6 on thepermanently magnetized transfer surface 2 of the roller 1 may be moreaccurately controlled by utilizing stripper blades placed at theappropriate distance from the transfer roller 1 in order to carefullyskim off any excess powder thereby obtaining the required thickness ofthe printing powder 6. With this arrangement any surface irregularitiestend to be eliminated and the printing powder 6 is therefore tended tobe more evenly distributed on thesurface 2 of the roller 1. It musthowever be noted that the amount of printing powder skimmed off must becarefully controlled in order to ensure that the permanently magnetizedsurface 2 of the roller 1 does not affect the latent magnetic image onthe recording surface 4 of the print drum 5.

erase unit 21 to be made viewable. The band 15 which is guided byrollers 16 and 18 and driven by means of the roller 16 which is rotatedin the direction of the arrow J by a drive shaft 17 attached thereto bymeans not shown in the drawing may be similar to that of magnetic tapesused in conventional recording apparatus, for example a supporting bandof an insulating and non-magnetic material such as a polymer film,coated on the surface 20 with a thin film of magnetic material such asiron oxide or a nickel cobalt alloy. 1

The colour of the printing powder is preferably dark to provide thehighest possible contrast with the surface 20; it is preferred toprovide the band with a white, matt silver or reflecting coating whichneed only be a few microns thick, in order to increase the contrast andhence the legibility of the displayed image or images. The surface 20 ofthe band 15 may be illuminated when in the display position by means ofa light source. It willbe seen from the drawing that the magneticrecording head unit 19 operates on the opposite surface of the Incertain instances, for example when lengthy storage of the magneticimage on the recording surface 4 is required, it may prove necessary tocontrol the relative strengths of the magnetic fields of the transfersurface 2 and the magnetic images on the recording sur-v face 4,'sincethe demagnetization effect of the transfer surface 2 will become veryimportant when lengthy band 15 from the surfaceto which the powder isapplied. This arrangement is preferred in order to reduce thepossibility of abrasion of the contrasting surface by the recordinghead, but since the magnetic coating on the film must then be on theside remote from the magnetic recording head the polymer film itselfmust be no more than about 0.001 inches thick. In order to achieve amore durable band 15, the supporting band of insulating and non-magneticmaterial may be replaced by a thin non-magnetic metal supporting bandfor example of beryllium copper.

\ It should be noted that the electrographic printing apparatus'anddisplay apparatus shown in the drawings according to FIGS. 1 -to 3 maybe combined as a single unit to provide both the display facilities anda permanent copy of the display either continuously or at will.

The transfer powdering devices outlined in preceding paragraphs areprimarily for use with horizontal recording surfaces, when it isrequired to powder a vertical recording surface utilizing this principlethen the transfer powdering device may take the form shown in thedrawing according to FIG. 4 which diagrammaticaly. illustrates asectioned front view of an electrographic printing and/or displayapparatus and a transfer powdering device. This transfer powderingdevice basically consists of a bevelled disc 22 i.e., frustoconical inform interposed between the powder reservoir 3 and the verticalrecording surface 25 of the machines print drum or band 24. j

The bevelled surface 26 of the disc 22 is permanently magnetised in amanner such as to collect thereon an evenly distributed layer of themagnetic printing powder 6 contained in the reservoir 3 when it isrotated therein in the direction of arrow K by a drive shaft 23 which isattached to the disc 1 by means not shown in FIG. 4. All operatingconsiderations etcetera which apply to the transfer powdering devicesoutlined in preceding paragraphs with reference to FIGS. 1 and 3 alsoapply to the transfer powdering device of FIG. 4.

The powder formulation which is utilized in the apparatus outlined inpreceding paragraphs includes a magnetic powder such as magnesium-nickelferrite or nickel-zinc ferrite which have a dielectric property that maybe utilized in the magnetic powder transfer process. For example, therotatable surface of either the band or roller 1 (FIG. 1), the rollers1, 1A and 1B (FIG. 2), theband (FIG. 3) or the bevelled disc 22 (FIG. 4)may be arranged such that -it can be charged electrostatically therebycausing the magnetic powder to be attracted thereto due to itsdielectric properties to form an evenly distributedlayer of powderthereon. When the apparatus is to be operated in a printing mode themagnetic powder, as previously stated, is coated with a thermal fixingagent, for example, resin or wax so that the powdered image may befixed? by the application of heat subsequent to the formation of thepowdered image therefore the dielectric properties of the fixing agentassists in the attraction of the magnetic powder particles to theelectrostatically charged rotatable surface.

With this arrangement the problems outlined in preceding paragraphswhich arise due to the demagnetising effects on the latent magneticimage or images by a permanently magnetised rotatable surface no longerexist and the displacement of the rotatable surface relative to therecording surface is no longer critical.

When the latent image or images on the recording surface of the printdrum or band are formed electrostatically, the recording surface will beof a material such as a dielectric material and the powder formula tionwill include .a dielectric powder or ferro electric powder, for examplebarium titanate to form the image or pattern. Therefore with thisarrangement of the apparatus the magnetic or dielectric properties ofthe powder may be utilized to effect the powder transfer i.e., therotatable surface may be either permanently magnetised orelectrostatically charged.

The fluid powder bed or the fluid powder bed and the transfer powderingdevice may be replaced by an impeller powdering device such as the onesillustrated in the drawings according to FIGS. 5 and 7.

Referring to FIG. 5 part of either the electrographic printing apparatusshown in the drawing according to FIG. 1 or 2 or the electrographicdisplay apparatus shown in the drawing according to FIG. 3 and animpeller powdering device are diagrammaticallyillustrated therein in asectioned front view. The impeller powdering device which is suitablefor use with horizontal recording surfaces basically consists of arotating impeller 27 having its axis parallel to the surface 28 whichmay be the recording surface of a print drum or the permanentlymagnetised or electrostatically charged transfer surface of a transferpowdering device, which p is rotated in the direction of the arrow G bya drive shaft 29 attached thereto by means not shown in FIG. 5.

The impeller 27 which is partially submerged in the printing powder 6contained within a container 30, is of a length at least equal to thelength of the surface 28 and is provided by an appropriate length of rodwith a very coarse gear tooth from 31 cut along its length. Whilst theimpeller shown in the drawing according to FIG. 5 is provided with agear tooth form 31, other forms would work equally well, for example therod may be formed along its length in a manner as shown in the drawingaccording to FIG. 6 which illustrates a cross-sectional end view of theimpeller. Referring to FIG. 6, the tooth form which is cut along thelength of the rod is, as can be seen from the drawing, of a special formin order to facilitate the collection of the printing powder 6 when theimpeller is rotated therein in the direction of the arrow G. Each of theteeth 32 are shaped so as to provide recesses 33 which are of a formsuch that they are capable of collecting a larger quantity of theprinting powder 6 than would be the case with the coarse gear tooth form31 shown in the drawing according to FIG. 5.

The teeth formed on the impeller 27 act as paddles in that as theimpeller 27 is rotated theycollect the printing powder and throw itvertically upwards towards the surface 28 thereby forming along thelength of the impeller 27 an arcuate sheet of powder.

The surface 28 is placed as shown in the drawing at apoint at the top ofthe arc where the powder and surface are moving in the same directionand relatively at zero velocity with respect to each other therefore theimpact force of the powder to the surface 28 is a minimum.

Printing powder is attracted to either the latent image or thepermanently magnetised or electrostatically charged surface dependingwhether the surface 28 is considered to be the recording surface of aprint drum or the permanently magnetised or electrostatically chargedsurface of a transfer powdering device to form as the case may be eithera visible image or an evenly distributed layer of the printing powder.

In order to produce an arcuate sheet of printing powder of constantcross-section along the whole length of the surface 28 a deflector plate34 is provided which is situated under the leading edge of the impelleralong its whole length and which is secured to the container 30 by meansnot shown in the drawing and supported therein at one end by means of asupport member 35. The other end of the deflector plate 34 is providedwith a number of adjustable members 36 which are spaced at regularintervals along the whole length of the deflector plate 34 and which aresupported by a member 37 which is attached to the container 30 by meansnot shown in the drawing. Each of the adjustable members 36 basicallyconsist of a female threaded bush 38 which is secured to the deflectorplate 34 and an adjusting screw 39 which screws into the bush 38, theheadless end of the screw 39 being supported by the member 37.

Adjustment of the screws 39 cause the gap between the deflector plate 34and the leading edge of the impeller 27 to be varied thus by selectiveadjustment of the screws 39 the height of the arcuate sheet of printingpowder may be adjusted at any point along the whole length of thesurface 28 thereby ensuring that the printing powder contacts all partsof the surface 28 situated directly above the top of or the highestpoint of the arcuate sheet of printing powder.

Variation of the speed of the impeller 27 allows adjustment of the meanheight of the arcuate sheet of printing powder.

Referring to FIG. 7 part of an electrographic printing and/or displayapparatus and another impeller powdering device are diagrammaticallyillustrated therein in a sectioned front view. This impeller powderingdevice which is suitable for use with vertical recording surfacesbasically consists of a shaft 40 for example 0.25 inches diameter with aspiral flute 41 (approximately t.p.i.) which is rotated by means notshown in the drawing in the direction of the arrow H within a closefitting sleeve 42 having a narrow slit 43 for example 0.032 inch wide inthe wall thereof which is parallel to the axis of the shaft 40. Thesleeve 42 is mounted vertically within a printing powder reservoir 44,for example any one of the fluid powder beds outlined in our copendingBritish Patent Application No. 56619/66 and secured thereto by means ofa mounting bracket 46.

The skirted end 45 of the sleeve 42 is suspended in the printing powder6 and the slit 43 is situated adjacent to a band or roller 47 which maybe the printing drum of an electrographic printing and/or displaymachine or the band or roller which forms part of a transfer powderingdevice.

The shaft 40 is rotated at a speed of the order of 1,500 rpm. and thedirection of the shaft rotation is such that the printing powder isdrawn up the tube 42 by means of the spiral flute 41 and due tocentrifugal action a controlled amount of printing powder 49 is ejectedfrom the slit 43 onto the surface 48 of the band or roller 47. Theskirting at the end 45 of the tube 42 ensures that a constant amount ofprinting powder is drawn up by the action of the rotating shaft 40.

Whilst the arrangement of FIG. 7 utilises a fluid powder bed, a powderbed having deflectors i.e., as shown in the drawing according to FIG. 5may be utilised providing a powder feed mechanism is connected theretoto ensure that an adequate powder level is maintained which issufficient to ensure efficient operation i.e. the skirted end 45 of thesleeve 42 is always covered with the printing powder 6. 1

It should be noted that a horizontal impeller having an action similarto the one described in preceding parwidth being 0.1 inches, a deflectorplate would be used to deflect the powder onto the recording surface andthe impeller speed and deflector system would need to be arranged suchthat the mean height of the sheet of printing powder thrown upwards bythe action'of the impeller is sufficient to powder the requisite partsof the recording surface.

FIG. 8 diagrammatically illustrates aplan view of an electrographicdisplay apparatus which utilises a transfer and/or impeller powderingdevice. The display appara'tus shown in FIG. 8 includes a band 50 whichis guided in a closed loopv by a series of rollers 51, a guide roller 52and a drive roller 53 for example a sprocket roller, the sprockets ofwhich engage with holes provided at the outer edges of the band 50. Amotor 54 drives the sprocket roller 53 and thereby the tape loop,

either continuously or under the control of input signals.

The correct tension of the band 50 is regulated by means of a roller 56which is rotatable mounted at one end of a pivot arm 57, the other endof this arm is pivoted at the position 58. The pivot arm 57 is attachedto a rigid support block 60 via a variable spring tension unit 59 bymeans of which the band tension is regulated.

A recording head is placed adjacent to the band 50 so that whenenergised by suitable electrical signals the head forms a pattern in theband as the latter moves in the-direction of the arrows X. This patterncorresponds to the visible image to be presented.

The band then passes through a powdering device 61 which may be providedby any one of the transfer or impeller powdering device outlined inpreceding paragraphs which aresuitable for powdering a verticalrecording surface, for example, the transfer powdering device shown inthe drawing according to FIG. 4. The powdering device 61 applies to thesurface of the band 50 a finely divided powder whose colour providesadequate contrast with the background of the band. The

- powder adheres to the band 50 in a pattern corresponding to thepattern formed by the recording head 55, and renders this patternvisible and suitable for direct viewlng.

The band 50 then passes to a display position where it can be viewed inthe direction of the arrow Y through a window 63 in the casing 64 of theapparatus.

The tape loop 50-completes its circuit by passing through a cleansingstage 62 where the printing powder is removed. It then returns to therecording head 55; depending on the mode of operation of the latter, apreliminary erase process, indicated in the drawing according to FIG.8-by an erase head 65, may or may not be required.

The apparatus therefore provides a means by which information receivedover an electrical input line can be rendered visible. The terminformation is here used in its widest sense and the apparatus may beused equally to display alphabetical or numerical matter, pictorialmatter such as graphs, diagrams or maps, or facsimile. In most casescode conversion will be required in the driving circuits of therecording head 55 in order to create the pattern corresponding, forexample, to a numerical input signal; on the other hand with certainforms of head and with an input derived from the line by line scanningof an original the input signals may be applied directly to the elementsof the recording head 55.

If a permanent record of the displayed information is required eithercontinuously or at will, then the apparatus shown in the drawingaccording to FIG. 8 may be modified to inlcude this facility by addingthe printing arrangement outlined in preceding paragraphs with referenceto the drawing according to FIG. 1. The printing arrangement would needto be interposed between the viewing stage and'the cleansing stage.

It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only and is not tobe considered as a limitawherein latent magnetic charge patterns createdon said recording surface are developed into visible images, a magneticpowder developing arrangement including a permanently magnetizedrotating surface, positioned proximate to the recording surface and inoperative contact with a source of magnetic developing powder particles,for evenly transferring from said source the magnetic particles to saidlatent magnetic image, developing same substantially independent ofinterference to the magnetic image by the magnetism of said rotatingsurface; said recording and'rotatable surfaces being separated one fromthe other by an amount which is dependent on the relative strengths ofthe magnetic field associated with said latent magnetic image and saidpermanently magnetized rotatable surface; means for regulating thethickness of the layer of faces are to said recording surface. l

1. In a printing and/or display apparatus, the combination comprising: abody having a recording surface wherein latent magnetic charge patternscreated on said recording surface are developed into visible images, amagnetic powder developing arrangement including a permanentlymagnetized rotating surface, positioned proximate to the recordingsurface and in operative contact with a source of magnetic developingpowder particles, for evenly transferring from said source the magneticparticles to said latent magnetic image, developing same substantiallyindependent of interference to the magnetic image by the magnetism ofsaid rotating surface; said recording and rotatable surfaces beingseparated one from the other by an amount which is dependent on therelative strengths of the magnetic field associated with said latentmagnetic image and said permanently magnetized rotatable surface; meansfor regulating the thickness of the layer of magnetic powder applied tothe recording surface, said regulating means including a plurality ofdrums which are interposed between the recording surface of saidapparatus and said source of magnetic powder, wherein the surfaces ofsaid plurality of drums are permanently magnetized, the adjacentrotatable surfaces of said drums being spaced apart from one another byan amount which decreases the nearer said adjacent surfaces are to saidrecording surface.